On the Terminal Effectiveness of Small-Arms Ammunition
© Anthony G Williams
Much has been written about the terminal effectiveness of the ammunition used in pistols and sub-machine guns (including the new breed of personal defence weapons, or PDWs) plus that used in assault rifles and light machine guns (which may often use the same rounds). Terminal effectiveness is primarily taken to mean the speed with which human targets can be expected to be put out of action by a hit in the torso. However, the conventional arguments tend to focus on a narrow aspect of effectiveness. This article argues that a wider definition is more appropriate and that this could lead to some different conclusions.
The wounding mechanism
The aspect of effectiveness which is usually considered is the size of the permanent wound channel which can be expected to be created by the bullet as it passes through the target. The reason for this is probably that it is the one aspect of wounding which is easily reproducible in scientific testing, by firing bullets into blocks of ballistic gel designed to reproduce the characteristics of flesh. This is used as a proxy for speed of incapacitation since, other things being equal, the bigger the wound channel, the faster the rate of incapacitation. The size of the wound channel is a function of the calibre, weight, impact velocity, shape and construction of the bullet.
There is no problem with this as long as it is remembered that other things are rarely equal. A range of other factors will come into play to determine the actual speed of incapacitation when an individual is shot. The first of these is, of course, the exact path which the bullet takes. The human body is not an homogenous block of gel, it is made of various constituents – flesh, organs, bones, blood vessels – which react in different ways to being struck. The obvious example is the central nervous system; the brain and upper spine. A hit in this area will almost always result in instant incapacitation and death. A hit which damages a major blood vessel may also result in incapacitation within a few seconds through a drop in blood pressure due to sudden and severe blood loss, followed by death a little later. Severe damage to other major organs may also be fatal after a period of time, but may not cause prompt incapacitation. Bullets which hit bone may cause secondary damage by driving bone fragments through the body. But those which pass only through muscle may cause little in the way of incapacitation.
One aspect of the effectiveness of bullets on the target is that the wounding mechanisms vary, and some are more reliable than others. The simplest way to produce a large wound channel is to hit the target with a large bullet, with enough energy to ensure that it penetrates right through (but no more, as surplus energy will be wasted on the background scenery); this will reliably produce a large wound channel, but such ammunition is heavy and also produces heavy recoil compared with smaller-calibre rounds. Normal practice in modern assault rifle / LMG ammunition is therefore to use a small calibre, high velocity (SCHV) cartridge such as the NATO 5.56x45 or the Russian 5.45x39. Like all pointed bullets, these are unstable in flesh and will turn over to travel base-first, this action magnifying the size of the wound. The stresses of this turning movement may also cause the bullet to break into two or more pieces depending on its design, which also adds to the wounding effect. The problem is that neither the overturning (popularly known as "tumbling") nor the break-up effects are entirely consistent, which (presumably along with the varying physical / psychological states of the targets described below) leads to very conflicting accounts of the effectiveness of these SCHV rounds. Sometimes they are reportedly devastating, sometimes ineffective.
It is worth emphasising at this point that it requires less power to inflict a fatal injury than it does to cause prompt incapacitation. Any hit which puts the target out of action quickly (which is the desired result) will have done such serious damage that subsequent death is highly probable unless very rapid medical aid is provided (and sometimes even when it is). It is therefore obvious that the oft-repeated myth that military ammunition is "designed to wound rather than kill" makes no logical sense.
Another vital issue in determining actual effectiveness is the physical and psychological state of the target. A man who is relaxed and at ease may collapse with shock on being shot, but the same man in a state of intense, adrenaline-driven excitement (such as might be expected in close combat) may not even realise that he has been hit. The same effect can be achieved by the use of certain drugs. There are countless anecdotal incidents of men who have suffered the most appalling – and ultimately fatal – injuries in battle continuing to fight aggressively for some time before collapsing.
A further aspect of terminal effectiveness is the level of protection enjoyed by the target. Body armour has now become standard issue in major armies for troops in combat, and its use can be expected to spread to more irregular forces, as does all affordable military technology which is deemed to provide an advantage. There are, of course, different grades of body armour, the weakest of which may only stop pistol ammunition, but the strongest can block armour-piercing rifle bullets. The more effective it is, however, the higher the penalty to the user in terms of its weight and bulk (not to mention heat build-up in high-temperature regions), so good armour has a negative effect on agility and endurance.
Perhaps the most problematic issue facing major armies today is to decide what level of armour its infantry weapons should be designed to defeat. To ignore this issue on the grounds that current opponents rarely use body armour will be to invite serious problems if – or rather when – this circumstance alters in the future.
It isn't just formal body armour which can affect terminal effectiveness; equipment worn on the person can do this (for example, loaded ammunition magazines in chest pouches). Barriers which the target may hide behind can also be penetrated by some ammunition much better than others (typically, large bullets at moderate velocities perform better in barrier penetration than small ones at high velocity, which partly accounts for the resurgence of interest in 7.62x51 calibre rifles in the US Army).
Taking all of the above issues into account it can be seen that, for the most part, the ballistic gel tests of ammunition do indeed provide a reasonably good idea of their likely effectiveness against unprotected targets. However, ammunition which is designed to penetrate armour is likely to have entirely different characteristics, which will undoubtedly cause those specifying these characteristics a considerable headache. Perhaps the answer will be to acquire two different natures of ammunition, one optimised for use against soft targets, one to penetrate armour (possibly using saboted tungsten sub-calibre bullets). The freedom to do this will however be limited by the need to ensure that both types work reliably in the same gun, unless different weapons are to be issued to meet the different demands.
The key ingredient: hit probability
All of the above is concerned with the effectiveness of those bullets which hit the target. There are, however, several other factors concerning ammunition and weapon design which have a bearing on a more fundamental issue; hitting the target in the first place. When it is realised that many thousands of rounds are fired for each hit achieved in battle, the importance of this is clear.
The three main aspects of ammunition which affect the hit probability are trajectory, recoil and weight. A flat-trajectory bullet, with a short flight time, facilitates aiming when the exact range is not known, as well as simplifying aiming at moving targets. To achieve this requires a high muzzle velocity, which (for any given calibre and bullet weight) means a bigger and heavier cartridge case, generating more recoil. This causes other problems, for three reasons: first, the heavier the ammunition, the less can be carried (and infantry in particularly are always grossly overloaded); second, heavier recoil increases the recovery time between aimed shots, plus makes it much more difficult to control automatic fire; and third, heavier recoil may also have a detrimental effect on acquiring shooting skills as it is more likely to cause the shooter to flinch while practising. This last point is particularly an issue when considering how best to arm non-infantry troops who only need a personal defence weapon in case of emergency, as they are likely to devote far less time to acquiring and practising those skills.
Gun design also has a major effect on hit probability. The most obvious difference is between pistols which are only held at one point (the grip) and those which are held at three points – pistol grip, shoulder stock and foregrip – and are therefore far easier to hold steady in the aim and to control when firing. This difference is immense: in the hands of average soldiers in combat, a stocked weapon is likely to match the hit probability of a pistol at something like ten times the range. In fact, it has long been observed that pistols are almost useless in combat except in the hands of highly-trained special forces, and may be a greater threat to friends than to enemies. It is therefore surprising that there has recently been a resurgence of interest in their use in the US Army, in the context of close-quarter urban fighting. It has to be asked, however, whether that is simply because their rifles – even the carbines – are rather long by comparison with more modern bullpup designs, and a pistol is the only alternative approved for service. Either a more compact rifle, or a purpose-designed close-quarter weapon (SMG/PDW), would probably provide a better answer.
Sights are another important factor. Small arms are increasingly being fitted with optical sights which can have a very beneficial effect on hit probability by speeding up target acquisition. The use of telescopic sights at long range is of course well-established, but has spread downwards from sniper rifles to assault rifles. At close range where the speed of aiming is the top priority, too high a magnification can hinder rapid target acquisition. Zero magnification holographic red dot sights are ideal for this purpose: the user does not need to place the eye accurately behind the sight as it is parallax-compensated, and it is best used with both eyes open to increase the field of view. Such sights are also smaller and lighter than telescopes, adding to their suitability for PDWs in particular. Sights which can be switched between 1x and 4x magnification are now available for assault rifles, which provides the best of both worlds at some cost in bulk and weight.
This brings me to a contentious issue: the use of automatic fire in rifles and PDWs. There are those who argue very strongly that, except in very specific circumstances (mainly, short-range suppressive fire), the use of the auto switch is simply a waste of ammunition: a better hit probability is achieved by aimed semi-automatic fire. There are others who observe that most hits in fire-fights have been achieved largely by chance, because the average soldier in the heat of combat is not in the best state to take cool and precise aim, and what improves the hit probability is getting as many rounds in the target area in the shortest possible time. They point out that if nothing else, a high volume of fire is more likely to keep the enemies' heads down and give the soldier a breathing space to recover. It is unlikely that this debate will ever be resolved, because the best approach evidently varies depending on the circumstances and on the individual soldier.
What clearly has the greatest effect on hit probability – and therefore on terminal effectiveness – has nothing to do with equipment or ammunition, however: it is training and practice. A soldier should be so well-drilled in the process of aiming and firing that he does not need to think about it; in the panic of close-combat, muscle-memory should take over and do the job for him. Modern training methods, including the use of non-lethal marker ammunition as made by Simunition and UTM for man-on-man practice, enables these skills to be polished to a greater degree than ever before. The problem is finding the time and money to do this properly; and in the case of non-infantry personnel, to ensure that they give it sufficient priority.
Weapon effectiveness is a complex subject involving much wider issues than just what happens when a bullet hits a body. All of the factors discussed above need to be taken into account in determining the most effective weapons and ammunition to issue to soldiers in particular roles. The ideal military rifle will be compact enough for use at short range but also effective at long range (which in effect means using the same ammunition as the squad LMG). The ammunition will be powerful enough to reach out to long range, and to reliably inflict severe wounds at short range, plus be effective (possibly in different loadings) against both armoured and unarmoured personnel. On the other hand, it must be reasonably light and compact and generate controllable recoil. There will never be agreement on this issue, but a cartridge intermediate in size and power between the current 5.56mm and 7.62mm NATO rounds would probably be the best compromise for most purposes, with the option to fire saboted bullets against armour.
Much the same can be said of the ammunition for PDWs, with the added proviso that since the weapon needs to be light and compact enough to be carried on the person so that it is instantly available at all times, the ammunition needs to be more compact and less powerful. Small enough, in fact, to be fitted into a pistol handgrip. It will therefore be much more problematic to achieve a high level of terminal effectiveness, and a lesser standard than the rifle/MG round must be accepted. The problems are magnified by the radically different characteristics needed to deal with unarmoured and armoured targets (although it is unrealistic to expect PDW ammunition ever to penetrate the higher grades of armour). What calibre PDW ammunition might be is therefore even more controversial than with rifle/MG cartridges.
An emphasis on penetrating armour led to the development of the small-calibre, high-velocity HK 4.6x30 and FN 5.7x28 cartridges, but there are concerns that these cannot offer enough effectiveness against unprotected personnel. On the other hand, the traditional large-calibre low-velocity pistol rounds are useless against almost all grades of armour, and their heavier recoil makes them more difficult to control in burst fire. At the moment, the Russian composite-cored 9mm AP bullets seem like a good compromise, with the option of moving to a round like the 6.5x25 CBJ should armour become more of a problem.
The difficulty with achieving good terminal effectiveness with a PDW round have led some to argue that the whole concept is a waste of time: that everyone, regardless of role, should carry the standard rifle. The risk with this is that non-infantry soldiers, concerned with their primary roles, will stow the rifle somewhere out of the way, where it may not be available when required.
As is usually the case, it will probably take extensive combat experience to determine the value of the new generation of compact SMGs/PDWs. Compared with rifles, whether their reduced terminal effectiveness is adequately compensated for by the ready available of the weapons (their compactness allowing them to be holstered). And compared with pistols, whether their much higher hit probability, resulting from a steadier aiming position and a controllable burst-fire capability, compensates for their greater bulk.
However ineffective a 4.6mm bullet may be felt to be, a hit with one of those is indubitably more effective than a miss with a pistol, or a failure to fire a rifle that just wasn't to hand.